1. Field of the Invention
The present invention relates to an optical information reproducing method and an optical information reproducing apparatus which are used for optical information recording media such as a magneto-optical disk, a compact disk (CD), and a CD-R, and in particular, is suitable for an optical magnetic reproducing method and an optical magnetic reproducing apparatus which reproduce information by utilizing a photo-electro-magnetic effect.
2. Related Background Art
Up to now in a record/reproduction system by optical information recording media, such as a magneto-optical disk, a compact disk, and a CD-R, it is known that a waveform deviation arises in a recorded signal or a reproduced signal according to the characteristics of a medium. The outline of waveform deviation will be described referring to FIGS. 41A and 41B. FIG. 41A shows a reproduced waveform and FIG. 41B shows decoded data. Here, it is assumed that a reproduced waveform is sampled at a leading edge of a clock in a reproduction system. A mark “X” in FIG. 41A denotes a sampling signal. If a waveform deviation arises due to the characteristics of a medium, a leading edge location is changed minutely due to the waveform deviation shown by a broken line when a signal changes from a level “H” to a level “L” at the time k1 in FIG. 41A. When the reproduced waveform where the waveform deviation arises is sampled, it becomes a factor of degrading a PLL loop and a decoder that are subsequent stages.
On the other hand, for example, Japanese Patent Application Laid-Open No. 10-50000 discloses a method of performing a data detection determination after adding a predetermined positive offset value to reproduced data at a turning point where a level shift from the level “H” to the level “L”. In addition, Japanese Patent Application Laid-Open No. 05-197957 discloses a method of compensating a waveform deviation at the time of recording by measuring record pulse width, etc. at the time of information record, and controlling a leading edge location.
By the way, in a PLL loop (data PLL) based on the sampled data of a reproduced signal, incorrect detection arises in a phase error signal by the waveform deviation. FIG. 26 shows the outline of the phase error detection in data PLL. In the data PLL, a phase error is obtained on the basis of the sampled data in an edge section of a reproduced signal. In FIGS. 42A to 42D, numeral 920 denotes a reproduced signal, and a black circle “●” and an open circle “◯” are sampled data with a clock.
FIG. 42A shows a case where the phase of the clock delays from the phase of the reproduced signal, and a phase error is outputted as a positive value. FIG. 42B shows a case where the phase of the clock is in conformity with the phase of the reproduced signal, and a phase error becomes zero. FIG. 42C shows a case where the phase of the clock leads the phase of the reproduced signal, and a phase error is outputted as a negative value. The PLL loop performs controls on the basis of this phase error signal. However, if waveform deviation 930 to the reproduced signal 920 arises as shown in FIG. 42D, a phase error detected in an edge section becomes a value shown by a black square “▪” and an error value is generated.
However, the above-described method of Japanese Patent Application Laid-Open No. 10-50000 in which only a level changing point is referred cannot treat, for example, a case where an amount of a waveform deviation changes depending on a waveform pattern to the changing point. Here, if a section of the level “H” is referred to as a mark, and a section of the level “L” is referred to as a space. If fluctuating the amount of a waveform deviation depending on an interval of the mark and space, etc., it is not possible to obtain a desired effect unless a correction amount of a waveform deviation is set adaptively according to the interval (record mark length) of the mark or space.
Furthermore, the method of compensating a waveform deviation at the time of record that is disclosed in Japanese Patent Application Laid-Open No. 05-197957 has a large possibility of generating bit droppage, etc. by the influence of compensation by a record pulse below the shortest mark length if the shortest mark length is shortened for a high densification. In addition, since the edge section of a reproduced signal is used in the data PLL, appropriate correction of the record mark length becomes necessary for achieving desired performance. Therefore, correction by a fixed amount of correction at a changing point of a level that is disclosed in the above-described Japanese Patent Application Laid-Open No. 10-50000 could not treat the waveform deviation of a reproduced signal.